Method of alloying gaseous materials with metals



Dec. 23, 1958 H. o. BEAVER, JR 2,865,736

METHOD OF ALLOYING GASEOUS MATERIALS WITH METALS Filed Feb. 8, 1956METHOD OF ALLUYING GASEOUS MATERIALS Wl'iH METALS Howard 0. Beaver, in,Reading, Pa,

Carpenter Steel Company, of New Jersey assignor to The Reading, Pa., acorporation This invention relates to a novel and improved method forintroducing alloying elements or compounds in gaseous form, such asnitrogen gas, into alloys, such as the steels, and particularly theaustenitic steels.

In the commercial production of austenitic steels, it has been the priorpractice to incorporate nitrogen into the molten steel in combined form,such as a nitride, a nitrogen-containing ferroalloy, nitrided ironpowder, etc. This practice leaves much to be desired, for it isextremely difficult to obtain a steel containing more than about 0.3%nitrogen using this method. Also, these combined forms of nitrogen arecomparatively expensive and their use adds considerably to the cost ofthe final product. Not only is it difiicult to obtain an austeniticsteel having more than 0.3% nitrogen, but when a nitro-' genconcentration in excess of this value is obtained using the customaryprocedure of melting the alloying metals under an atmosphere of air, itis diflicult to cast sound and homogeneous ingots.

It is also known that the solubility of nitrogen in an alloy may beincreased by adjusting the composition of the base alloy to increase theconcentration of certain nitrogen-solubilizing elements, such aschromium and manganese. For example, the solubility of nitrogen inliquid iron containing only 16% chromium is 0.19% at 1600 C. and if 16%chromium and nickel are present the nitrogen solubility is reduced to0.16%.,

Liquid iron at this same temperature and which contains 16% chromium and10% manganese will dissolve 0.25% nitrogen. However, the amount ofnitrogen retained in steel which has solidified without porosity isusually less than the saturation concentrations in molten steels.Furthermore, even though the concentration of nitrogen-solubilizingelements in the basic alloy is substantially increased, only slightincreases innitrogen concentration are obtained. This technique is not apractical one since these nitrogen-solubilizing elements are alsoexpensive and often scarce and higher concentrations of these elementsmay impart undesirable properties to the resulting alloy.

In spite of these difliculties, substantial benefits are obtained byincreasing the nitrogen content of many alloys and a great deal ofresearch effort has been directed to attempts to obtain higher nitrogenvalues. Previous efforts to solve the problem have not providedsatisfactory results. Among the benefits resulting from theincorporation of nitrogen into alloys are those which may be obtained inthe stainless steels. Nitrogen serves as an excellent replacement fornickel in austenitic stainless steels. increases of a fraction of apercent in the nitrogen content will permit substantial reductions inthe amount of nickel used in the alloy and yet'provide an austeniticsteel having comparable properties. This permits a substantial savingsin the amount of the expensive and comparatively scarce nickel. Inchromium grades of stainless steel nitrogen imparts fine and even grainstructure, improved hardness, and greater strength and ductility atelevated temperatures. Nitrogen-bear- "ice 2 ing, 0.03% carbon, 18-8stainless steel (18% chromium and 8% nickel) will provide stress rupturestrengths in the l200-l500 F. temperature range which are comparablewith those 18-8 type stainless steels which also contain columbium.

It is an object of the present invention to provide an eflicient,reliable and economical process for introducing alloying elements ormaterials in gaseous form into alloys.

It is another object of this invention to efliciently, reliably andeconomically produce alloys, and particularly steels, such as theaustenitic steels containing chromium and/or manganese, which shallcontain a higher concentration of nitrogen than has heretofore beenpractical without incurring concomitant porosity or other undesirablecharacteristics.

It is an additional object to provide a cast alloy containing more thanthe normal saturated concentration of a gaseous alloying elementhomogeneously distributed throughout the casting Without voids beingpresent.

it is a further object of this invention to produce austenitic alloysmore economically in which nitrogen is employed in place of some of thenickel as an austeniticforming element.

Other objects and advantages of the invention will be apparent to thoseskilled in the art from the description which follows.

With these and other objects, features and advantages in view, theinvention will be described in greater detail by reference to theaccompanying figure of drawing which is a diagram of the melting furnaceand appurtenant apparatus employed in the process.

As employed throughout this application and the discussion whichfollows, the term alloying element is intended to refer to the elementper se, and to chemical compounds containing the element. Thus in thecase of sulfur the element may be introduced into the alloy as an oxideof sulfur, such as the compound sulfur dioxide.

In the discussion which follows, the invention will be describedprimarily in connection with the employment of gaseous nitrogen as thegaseous alloying element or material. However, it will be readilyunderstood that the process of the invention is applicable to otherelements or materials in gaseous form. For example, sulfur may beintroduced into an alloy in the form of a gaseous oxide of sulfur, suchas sulfur dioxide, employing the process of this invention.

Theprocess of this invention comprises a method for introducingeconomically, reproducibly and efiicie'ntly alloying materials ingaseous form, such as gaseous nitrogen, into alloys. In accordance withthis process the gaseous material is introduced over the base alloywhile the alloy is still in the molten state, or prior to melting thebase alloy, and maintained over the alloy while it is cast. Maintenanceof the gaseous alloying material over the alloy both while the latter isin the molten state and during casting and cooling constitutes anessential feature of this invention.

Preferably the atmosphere of air over the base alloy is removed aseffectively as possible before the alloying gaseous element is added.This may be accomplished by vacuum melting techniques with which the-artis familiar. Vacuum treatment removes most of the air and oxygen fromthe system and this assists in enhancing the solu bility of nitrogen orother alloying element in the alloy. High vacuum treatment and refiningof the alloy is preferred and pressures of about 10 microns of mercuryor less are most satisfactory, although higher pressures, such as about1 millimeter of mercury or higher, are also satisfactory. High vacuumtreatment is preferred to higher pressures at this stage of the processsince it permits more rapid alloying of larger concentrations of thegaseous alloying element into the base alloy during the subsequentstages of the process.

After this vacuum refining of the base alloy, a nitrided alloy, such asnitrided ferro-chromium or nitrided manganese, is desirably added if thegaseous alloying element sought to be added is nitrogen. At about thetime that the nitrided alloy is added, nitrogen gas is admitted into theevacuated system. Normally sufficient nitrogen, or other gaseousalloying element, is admitted to equalize atmospheric pressure (760 mm.mercury) although greater or lesser pressures are satisfactory.Preferably pressures in excess of 760 mm. of mercury are employed.Mixtures of gases containing nitrogen may be used but it is preferred touse substantially pure nitrogen since the solubility of the nitrogen inthe alloy is somewhat dependent on the partial pressure of nitrogen inthe system. The melting of the alloy is completed and any additionalalloying materials added, after which the molten alloy is cast bypouring it into a mold and permitting it to cool and solidify. Duringthe melting and casting operations the atmosphere of nitrogen ismaintained. This process permits the incorporation of nitrogen inamounts in excess of the amount added in the form of the nitrided alloyand in excess of the amounts heretofore possible by prior art processes.The period during which the system is under an atmosphere of nitrogengas may vary considerably, but short periods of from 6 to 35 minuteshave proven satisfactory.

As an alternative to the above described embodiment, the atmosphere inthe furnace over the cold, unmolten charge from which the base alloy isproduced may be evacuated before melting the components of the basealloy. The gaseous alloying element or material is then admitted intothe furnace until the pressure desirably reaches atmospheric pressure orhigher and the heat is 'then melted, alloyed, cast and solidified underthe atmosphere of the gaseous alloying element. Results similar to thoseof the previously described embodiment are obtained.

When alloying a base alloy with nitrogen gas, it is preferred, but notnecessary, to supply part of the nitrogen in solid form, such as anitrided alloy, as described hereinabove. Additions of the nitridedalloy along with gaseous nitrogen permit the alloying of largerconcentrations of nitrogen into the base alloy more rapidly andefliciently.

In the practice of the process of the invention it is desirable toremove from the atmosphere above the molten alloy and solidifyingcasting as much as possible of the oxygen, hydrogen and water vaporwhich are normally present in the air. Although these gases maytheoretically be considered as alloying elements, they generally impartundesirable properties to cast ferrous base alloys and it is thereforegenerally preferable to eliminate them from the system. By subjectingthe system to high vacuum and then introducing the gaseous alloyingelement in relatively pure form, the elimination of the undesirablegases may be achieved.

The process of the invention may be further illustrated by referring tothe diagram of the appended drawing. In the diagram, (H) is a standardvacuum furnace which houses a melting crucible (A) having a remotecontrol pouring device, a tiltable tray or bin (F) to charge thecrucible with nitrided alloy, and a mold (B) into which the molten alloymay be poured from the crucible and ingots 'cast. The heating of thecrucible may be effected by means of a high frequency electric generator(G). The vacuum furnace is evacuated by means of a difiusion pump (C) insequence with and augmenting a mechanical pump (D). The gas, such asnitrogen, is supplied by a tank (E) which is connected with the furnaceby a suitable pipe or tube. The important feature of this vacuum furnaceis that it permits retention of the nitrogen or other gas over the alloyboth while it is in the molten state and while it is being cast andcooled.

The process of the invention is applicable to ferrous and non-ferrousalloys of many types. Where the process is employed to introduce gaseousnitrogen as an alloying element, the invention is most advantageous forthe production of the stainless steels, and particularly the austeniticstainless steels, such as those containing chromium, nickel and/ ormanganese, each of which may or may not contain nickel as an alloyingelement. The invention is applicable to the treatment of so-called 18-8stainless steel, as well as straight chromium stainless steels.

It has been found that the nitrogen content which may be obtained in analloy by practicing the process of this invention will vary somewhatdepending upon several variables. The final concentration will bedependent upon the composition of the base alloy. Alloys containing ahigh total concentration of chromium and manganese tend to dissolvelarger percentages of nitrogen. The process of the invention isparticularly effective in increasing the nitrogen content of alloyshaving a high concentration of these elements and it has been possibleto produce stainless steel ingots containing these metals which willcontain about 0.6% to 0.7% nitrogen. The concentration of nitrogen isalso somewhat dependent upon the degree of refinement of the moltenmetal and the amount of nitrogen introduced into the molten alloy in theform of nitrided master alloys. In general, the amount of nitrogendissolved in the final alloy will increase when the nitrogen gasmaintained over the alloy during melting and casting is maintained athigher pressures. The amount of nitrogen dissolved in the cast finalalloy is also dependent upon the partial pressure of the nitrogen in theatmosphere over the alloy during melting and casting. The greater thedifferential between the partial pressure of the nitrogen in the metaland the nitrogen in the atmosphere above the metal, the greater is thetendency of the alloy to retain more nitrogen in solution. In most caseshigher nitrogen contents are achieved when the melt is held under anatmosphere of nitrogen for longer periods of time. The amount ofnitrogen dissolved will also depend upon the temperature of the moltenalloy.

The successful results achieved by the process of this invention arebelieved to stem primarily from the fact that the gaseous alloyingelement, such as nitrogen, is

.held in solution during solidification, whereas in the processes of theprior art, which do not cast the alloy under an atmosphere of thegaseous alloying element, there is a marked decrease in solubilityduring solidification. For example, nitrogen is more soluble in themolten alloy than in the solid alloy. The nitrogen atmosphere providesnot only a source of nitrogen to the alloy, but it assists inmaintaining the nitrogen in solution because of the pressure of thenitrogen over the solidifying mass of metal. By means of the process ofthe invention it is possible to consistently provide alloys containingat least 0.3% of nitrogen. One of the important features of theinvention is that it provides alloys containing more than the normalsaturated concentrations of the gaseous element. Yet the gaseousalloying element introduced is homogeneously dissolved throughout thecast alloy or ingot rather than as a rind or surface concentration ofthe gas. This provides a highly homogeneous casting. When castings havebeen analyzed to determine the concentration of the gaseous elementintroduced during the process, it has been found that the concentrationof the dissolved gas is substantially the same at all positions in thecasting, whether the sample originated from the surface or the center ofthe casting. The castings are also substantially free of voids.

In order more clearly to disclose the nature of the present invention,specific examples illustrating the proc ess will hereinafter bedescribed. It should be understood, however, that this is done solely byway of example and is intended neither to delineate the scope of theinvention nor limit the ambit of the appended claims. Unless otherwisestated, quantities of materials, including nitrogen and other alloyinggases, are referred to throughout this application in terms of percentby weight.

Examples 110 Examples 1 describe the introduction of gaseous nitrogeninto stainless steels in accordance with the process of this invention.Examples l7 describe the application of the process to molten steelscontaining chromium, nickel and manganese. Examples 8 and 9 describe theapplication to straight chromium stainless steels. Example 10 describesthe treatment of a chromium-nickel stainless steel which also contains1.2% aluminum. Each example is described in part by the table below.(The percentages of the elements contained in the molten base alloy andin the final cast alloy are given in the table. The difference betweenthe total percentages of the elements given and 100% is made up ofiron.)

Example 11 This example describes the introduction of sulfur in the formof sulfur dioxide into an alloy in accordance with the process of thisinvention.

A heat containing about 0.15% to 0.3% silicon, 0.013% sulfur, 0.15 to0.3% chromium and the remainder iron was charged into a vacuum furnace.The furnace was evacuated to a pressure of less than 10 microns ofmercury. The pressure was then increased to about 1 mm. of mercury byintroducing sulfur dioxide gas. The charge was melted and maintained inthis condition for about 2.3 hours. The pressure was then raised toabout 600 mm. of mercury with helium gas and maintained at this pressurefor about 0.15 hour. While under the combined atmosphere of helium andsulfur dioxide, final alloys of carbon and of manganese were added andthe melt was then cast by pouring into a mold and cooling. During thiscasting operation the pressure in the furnace remained substantially thesame. The final cast alloy was analyzed TABLE I Percent NitrogenPressure Time Bath Added as of Nitro- Un'lcr Mauga- Phos- Chro- Molyb-Final Ex. Nitrided gen Atmos- Nitrogen Carbon nese Silicon phorus Sulfurmium Nickel denum Nitrogen Ferrophere, mm. Pressure, Content Chromiummin.

Alloy Aluminum 2. 12

In each of these examples a base alloy containing iron and the otheralloying elements listed in the above table (with the exception ofnitrogen) were melted in the crucible of a vacuum furnace in accordancewith the standard practice in the art. The vacuum furnace shown at (H)of the appended diagram was then evacuated by means of the diffusionpump (C) and mechanical pump (D) connected in series. The pressure wasreduced to about 10 microns of mercury. In each example nitridedferro-chromium alloy in sufficient quantity to introduce into the alloythe percentage of nitrogen found in the second vertical column of thetable was introduced from the tiltable bin (F) into the melting crucible(A) which contained the molten base alloy. At about the same time,nitrogen gas was admitted into the vacuum furnace from (E) until thepressure of nitrogen was that listed in the table. The nitrogenatmosphere was maintained over the molten bath and the analyses of thealloys adjusted as needed and the molten alloys poured from the crucibleinto mold (B) and cooled. During the final melting and castingoperations the nitrogen atmosphere in the vacuum furnace was maintainedat substantially the same pressure. The time during which the bath wasunder nitrogen gas pressure in each example is stated in the table. Theresulting ingots of the cast alloys were analyzed and found to containthe percentage of nitrogen stated in the column at the righthand side ofeach example in the table. As an indication of the excellent resultsobtained by the use of this process Example 7 in the table above shouldbe noted. This cast alloy contained approximately 10% manganese and21.43% chromium and had a final nitrogen content of 0.67%. The castingwas perfectly sound without any indication of porosity.

and found to contain the following composition: 0.9% carbon, 1.6%manganese, 0.04% silicon, 0.004% phosphorus, 0.25% chromium, 0.02%nickel, less than 0.01% molybdenum, and the sulfur content was found tobe 0.251% at the surface and 0.262% in the center of the cast ingot. Theremainder of the alloy was iron.

The terms and expressions here employed are used as terms of descriptionand not of limitation, and it is not intended, in the use of such termsand expressions, to exclude any equivalents of the features shown anddescribed or portions thereof, but it is recognized that vanousmodifications are possible within the scope of the invention.

What is claimed is:

1. A process for introducing a gaseous alloying material into an alloy,which comprises withdrawing the atmosphere surrounding the molten alloy,introducing the gaseous material over the molten alloy and then castingthe molten alloy while in contact with the gaseous mate rial.

2. A process as defined by claim 1, wherein the gaseous alloyingmaterial is sulfur dioxide.

3. A process for introducing a nitrogen gas into a steel, whichcomprises withdrawing the atmosphere surrounding the molten steel,introducing nitrogen gas over the molten steel and then casting themolten steel while in contact with the nitrogen.

4. A process as defined by claim 3, wherein the steel is a stainlesssteel.

5. A process as defined by claim 3, wherein the steel is austeniticsteel.

6. A process as defined by claim 3, wherein a nitride is also introducedinto the molten steel to provide an additional source of alloyingnitrogen.

7. A process as defined by claim 3, wherein the gaseous nitrogen isintroduced until a pressure of about 1 atmosphere of nitrogen isobtained.

8. A process as defined by claim 6, wherein the nitrogen content of thefinal cast alloy is at least 0.3%.

9. A process as defined by claim 1, wherein the surrounding atmosphereis withdrawn by evacuating the system to a pressure of less than about 1mm. of mercury.

10. A process as defined by claim 1, wherein the surrounding atmosphereis Withdrawn by evacuating the system to a pressure of about 10 micronsof mercury.

, References Cited in the file of this patent UNITED STATES PATENTS

1. A PROCESS FOR INTRODUCING A GASEOUS ALLOYING MATERIAL INTO AN ALLOY,WHICH COMPRISES WITHDRAWING THE ATMOSPHERE SURROUNDING THE MOLTEN ALLOY,INTRODUCING THE GASEOUS MATERIAL OVER THE MOLTEN ALLOY AND THEN CASTINGTHE MOLTEN ALLY WHILE IN CONTACT WITH THE GASEOUS MATERIAL.